1. Field of the Invention
The present invention relates to semiconductor electronic devices including molybdenum oxide formed on substrates which consist of materials which are used in known semiconductor electronic devices.
More particularly, the present invention relates to field effect transistors, bipolar transistors, thyristors with a high breakdown voltage and hostile-environment electronic devices made up of new semiconductor which can solve difficult problems accompanying to such devices made up of known semiconductors with a large bandgap such as gallium nitride (GaN) and silicon carbide (SiC).
The present invention relates to also a new method to fabricate said electronic devices on substrates made of materials which have been used in usual electronic and photonic devices.
2. Related Background Art
Recently so-called power devices such as bipolar transistors, field effect transistors, and thyristors are used in various fields such as domestic electronic articles, cars, machine tools, and illumination. With increase of application, conversion and control of electric power with high efficiency and with a high speed are requested to power devices. Although power devices have been fabricated using silicon (Si) for a long time, limits of silicon devices are predicted. The limits come from the fact that the bandgap of silicon, about 1 electron volt (eV), is small. Research to realize power devices made up of semiconductors with large bandgaps, that is, so-called widegap semiconductors to overcome the limits has been widely done. In particular, development of power devices using gallium nitride (GaN) whose bandgap is about 3.43 eV or silicon carbide (SiC) whose bandgap is about 3.2 eV has been done extensively.
On the other hand, error or trouble of electronic devices due to noise which comes from the cosmic rays or cars and heat has been serious problems. It has been made clear that so-called hostile-environment devices which are proof against a severe environment with noise or heat should be made of semiconductors with large bandgaps. Development of electronic devices using GaN or SiC has been done from these points. However there are many additional problems to be solved to realize electronic devices made of GaN or SiC.
One of the most serious problem is that bulk crystal of GaN has not been obtained because an equilibrium vapor pressure of nitrogen is very high relative to that of gallium. Therefore, substrates made up of sapphire or silicon carbide (SiC) are used. GaN can not be formed directly on a sapphire substrate because there is lattice mismatch of 16% between sapphire and GaN. Therefore a buffer layer of aluminum nitride (AlN) is formed on a sapphire substrate before growth of GaN. AlN is resistive because it is difficult to dope impurities into AlN. Use of sapphire substrate in a device which includes multi-layers of semiconductor such as a bipolar transistor and a thyristor is very disadvantageous to their structures and fabrication process. On the other hand, SiC substrate is very expensive because bulk crystal of SiC can be grown at a very high temperature of 2200˜2400° C. GaN devices using SiC substrate or SiC devices are very expensive.
The problem to be solved realize a new device is to obtain a new substrate which replaces expensive substrate such as sapphire or SiC.
The second serious problem is to realize new devices which can be grown at a lower temperature than that at which GaN or SiC layers are formed. It is necessary to form layers of GaN or SiC at a temperature higher 1000° C. Large energy is necessary to form semiconductor layers at a high temperature. In addition, there are possibilities that atoms move between layers and a composition is disturbed or dopants move near the interface between layers.
The problems described above can be partly solved by using molybdenum oxide for such photonic devices. The inventor of the present invention discovered that high quality molybdenum oxide crystal has a bandgap larger than 3.2 eV and is very useful to be used in photonic and electronic devices (U.S. patent application Ser. No. 10/848,145 and Ser. No. 10/863,288).
In the patent applications described above, the facts and methods discovered by the inventor of the present invention are described. They are summarized as follows.
(i) High quality molybdenum oxide crystal has a bandgap of 3.45˜3.85 eV. The results were obtained by experiment for molybdenum oxide layer with a thickness larger than 10 μm grown by oxidation of a molybdenum plate with a purity of 99.99% in oxygen with a purity of 99.9995%. For example, molybdenum oxide formed by oxidation at 550° C. for 120 min had a bandgap of 3.66 eV. The reason why molybdenum oxide formed by the method invented by the inventor of the present invention had a bandgap larger than that previously reported is that it was high quality molybdenum oxide crystal having a thickness larger than that previously reported. A bandgap is affected by structure of a layer, that is, crystal or non-crystal, strain in the layer and purity.(ii) It was confirmed that molybdenum oxide formed by the present inventor's method is semiconductor also based on measurement of electronic properties.
However, in the patent applications described above, the molybdenum oxide crystal was formed by oxidation of a part of a metallic molybdenum plate. A part of a metallic molybdenum plate which was left without oxidation. Because the molybdenum plate was not crystal, some fabrication technologies such as cleavage could not used. Furthermore, precise control of a thickness of the molybdenum oxide layer was difficult when it was formed by oxidation of a molybdenum plate.
Therefore it is required to form a layer of semiconductor crystal having a bandgap larger than 3.2 eV on a new substrate which preferably consists of material which is used in known devices and preferably is crystal.
In the present patent, molybdenum oxide film can be formed on a substrate consisting of material which has been used in usual electronic and photonic devices. The most popular material is silicon. One of the most serious problem on a substrate can be solved by the present invention.
In the present invention, molybdenum oxide film can be formed on a substrate by a new method at a temperature lower than 850° C. Therefore, the second serious problem can be solved by the present invention.